1. Field of the Invention
This invention relates in general to an exhaust gas purifying system for an internal combustion engine, and more particularly to a secondary air injection compensating-system adapted to prevent uneven secondary air supply due to a large variation in air fuel ratio which occurs during acceleration, thereby effecting secondary air compensation with high accuracy.
2. Description of the Prior Art
In an internal combustion engine having an exhaust system provided with a catalytic device and a secondary air supply system positioned upstream thereof, it is desirable to control the secondary air fuel ratio to the theoretical raio (excess air ratio .lambda.= 1) in conformity with the operating condition. However, in the prior art system wherein secondary air supplied from its source is delivered, through the operation of an air by-pass valve responsive to the vacuum in the intake manifold, to a secondary air switching valve which is actuated by the manifold vacuum through a vacuum transmitting valve, there occur fluctuations in the amount of injected secondary air according to the position of the air switching valve so that difficulties have been encountered in accomplishing optimal secondary air compensation. Means have therefore been wanted to ensure sufficiently accurate secondary air supply to achieve the desired effect.
Particularly, at the time of engine acceleration, stepping on the accelerator pedal tends to cause a decrease in air fuel ratio which is attributable to the characteristics of the carburetor, and also tends to cause insufficient secondary air supply so that there may arise a phenomenon undesirable from the viewpoint of air pollution by exhaust gas. In order to normalize the air fuel ratio during acceleration, it is desirable to make an adequate increase in fuel flow in accordance with the acceleration as well as fuel supply from the accelerating system of the carburetor. This, however, has no relation with countermeasures concerning the secondary air system and is very difficult to practice in a suitable manner in view of the operating characteristics of the engine.